(1) Field of the Invention
The present invention relates to an inspection method for a soldering condition using X-ray imaging of electronic parts with mounted on a substrate by soldering with their leads so as to determine the soldering conditions of the leads soldered to the substrate based on a transmitted X-ray image in a preferable condition.
(2) Description of the Prior Art
With recent trends toward reduction in size and high density mounting of electronic parts on a printed circuit board, the use of electronic parts of the surface mounted device type (an electronic part which is mounted on a substrate by soldering its leads without using through holes) has gradually increased. In accordance with the above described tendency, a soldered joint of a lead on a surface of the substrate also has exhibited a gradual tendency to become small in size. As for a an inspection method for soldering condition of the small sized soldered joint, an inspection method for soldered joints using X-ray imaging as disclosed in JP-A-2-138855 (1990), for example, has gradually become widely used.
The inspecting method for a soldered portion using an X-ray imaging is briefly explained hereinafter.
When an X-ray is projected to a soldered portion, images corresponding to solder and leads in an X-ray image are detected darker than other portions because the solder and the leads have larger absorption coefficients for the X-ray than the substrate, and since the transmission path of the X-ray is longer in the solder and the leads than in other portions, the X-ray is absorbed more in the solder and the leads than in the other portions. Consequently, an excess or a lack of the solder can be determined by judging a condition of brightness or darkness of the X-ray image corresponding to the soldered portion. Especially, for electronic parts which are mounted on a surface of substrate by soldering their leads, whether the soldering condition is perfect or defective is clearly revealed in a quantity of the solder at a portion called a heel under the lead's standing portion. Accordingly, a defective condition of the soldering is easily determined by judging a condition of brightness or darkness in an X-ray image corresponding to the heel.
For an example, an electronic part having leads with a relatively large pitch size such as 1.27 mm pitch or 0.8 mm pitch has large size leads, and consequently a quantity of solder existing at a soldered joint between the lead and the surface of the substrate becomes inevitably large. And thickness of the existing solder in a perfect condition differs remarkably from the thickness of the existing solder in a defective condition. Accordingly, even if the standing portion of the lead is detected as an image superimposed on the image of the solder, the difference between the thickness of the solder in the perfect condition and in the defective condition appears relatively large in an X-ray image, and an effect of the standing portion of the lead becomes negligible in the judgement of the defective soldering condition.
However, in accordance with the current trend of high density mounting of electronic parts on a substrate, the pitch of the leads has become small and the quantity of the solder for soldering joints has decreased remarkably. For instance, a thickness of the soldering joint of an electronic part having 0.5 mm lead pitch is almost 1/2, and that of one having 0.3 mm lead pitch is almost 1/3, of the electronic part having 0.8 mm lead pitch. In accordance with the decrease in the thickness of the soldering joint as above described, the effect of the standing portion of the lead which is detected as an image superimposed on the image of the soldering joint is no longer negligible in the inspection of the soldered joint using X-ray imaging.
But, in accordance with the prior art inspecting method for a soldered joint disclosed in JP-A-2-138855 (1990), an X-ray is projected to the substrate from an oblique direction merely in order to avoid superimposing respective images of soldering joints at a front surface and a back surface of the substrate, and consequently consideration of an inclination of a standing portion of the lead and other factors is insufficient and a soldering condition of the soldered joint cannot be determined preferably.
Furthermore, detected wave shapes of X-ray images are indicated in FIGS. 11b and 11d for a case when the soldered joints exist only at the front surface of the substrate and an X-ray is projected to the soldered joints from above the substrate. A difference between the detected wave shape FIG. 11b for a perfect soldering condition which is shown in FIG. 11a and the detected wave shape in FIG. 11d for a defective soldering condition which is shown in FIG. 11b is scarcely visible. Accordingly, when a small soldering joint is inspected by the above described inspecting method, a transmitting path of the X-ray through the standing portion of the lead is longer than that at the other portion of the lead, and an image at the standing portion of the lead superimposes on an image at the soldering portion. Consequently, features showing the soldering condition are scarcely visible in the X-ray image at the standing portion of the lead, and the judgement of whether the soldering condition is perfect or defective becomes difficult.
Furthermore, as another prior-art method, an inspection method for a micronized soldering portion is disclosed in JP-A-4-105050 (1992). In accordance with the prior art, an X-ray is irradiated to a soldering portion of a lead of an electronic part mounted on a surface of a substrate while almost continuously changing an incident angle to the soldering portion, and subsequently an intensity signal of the penetrated X-ray is detected corresponding to each change of the incident angle, a curve which expresses an amount of the penetrated or absorbed X-ray which passes through a point of a heel portion of the lead and spreads radiately, as a function of an angle .theta. around the point of the heel portion is derived from the detected intensity signals, and the quality of the soldering portion of the lead is determined from the curve. However, the above prior art has a problem that the inspection method according to the prior art requires a long time for the inspection because the detection of the intensity signal of the penetrated X-ray must be performed several times even for one soldering portion of the lead, for instance, nine times if the incident angle .theta. of the X-ray varies from 40.degree. to 120.degree. and the intensity signal of the penetrated X-ray is detected at every 10.degree..